1. Field Of The Invention
The present invention relates to printer apparatus and, more particularly, to a lead screw driven, resiliently mounted drive nut and carriage assembly therefor.
2. Description Of The Prior Art
In lead screw driven printers, a print head is normally mounted on a suitable carriage which is driven reciprocally across the width dimension of a web, such as paper in roll stock form, or any other suitable record medium on which printing is to take place. The web is typically drawn over a rotatable platen with either frictional engagement with the latter or engagement with an associated sprocket wheel employed to effect controlled line feed advancement of the web.
The carriage is normally driven along a pair of guide rods aligned in parallel relationship with the lead screw. The carriage (and print head mounted thereon) is coupled to the lead screw by a threaded member, usually in the form of a drive nut.
With the lead screw normally driven by a reversible stepping motor, for example, the rotational displacement of the lead screw is translated through the drive nut into linear reciprocal displacement of the carriage (and print head). The direction in which the carriage is driven, and the speed of travel thereof, of course, is directly dependent on the direction and speed of rotation of the lead screw.
Such lead screw driven carriages are often employed in high speed printers, particularly of the dot matrix type. One such illustrative printer is disclosed in a commonly assigned and concurrently filed copending application of J. L. DeBoo-E. C. Feldy-H. S. Grear, Ser. No. 468,046, herein incorporated by reference.
While a power driven lead screw affords a number of advantages over belts or chains for driving carriage-mounted print heads in terms of simplicity, ruggedness, cost and maximum possible driving speed, they nevertheless have presented a number of troublesome problems heretofore. Specifically, because of the necessity of threads, unless stringent tolerances are adhered to in the manufacture of the lead screw and drive nut, there must normally be either some backlash allowed for therebetween, or a resilient drive nut employed in order to minimize the possibility of excessive frictional forces being established.
Attempts to go the route of manufacturing the lead screw and drive nut with stringent tolerances has proven to be impractical in practice for a number of reasons. First, the lead screw must necessarily extend across the entire width dimension of the printer, i.e., in parallel relationship with the platen and, as such, there is a tendency for the lead screw to inherently have or develop a slight bow which is most pronounced along the intermediate region thereof. Secondly, while the lead screw is normally mounted on precision ball bearings (or bushings), tolerance variations in the bearing mountings, as manufactured, or as positioned on supporting frame structure of the printer, invariably leads to slight, but normally troublesome misalignment between the lead screw and carriage guide rods. Thirdly, because of the size of the threads and the axial length of the lead screw, a precision machining operation, as distinguished from a conventional and simple cold rolling operation, to form the threads would prove prohibitive from a cost standpoint.
Accordingly, even if a conventional drive nut could be manufactured to threadably engage the lead screw in a very close fitting manner with negligible backlash, very high frictional forces would normally still develop not only between the lead screw and drive nut, but also between the lead screw and carriage guide rods. Such frictional forces would lead to excessive wear of the mating parts generating them, and could possibly overcome the driving torque of the stepping motor. In the latter case, the carriage would actually bind or lock-up on the guide rods. Such a condition, of course, could very possibly also seriously damage the stepping motor in many printers.
Equally important, however, is the fact that any non-uniform frictional forces, whether great enough to actually bind the carriage or not, would necessarily at least alter the speed at which the carriage is either continuously driven or stepped along the guide rods. Such unintended variations in carriage speed during printing cannot be tolerated, as there must be a very precisely correlated relationship between the firing of the print wires (or hammers) and the lateral position of the print head at each successive dot position along a given print line.
In an attempt to solve some of the foregoing problems, specially constructed, elongated drive nuts have been proposed and/or used heretofore wherein the central bore has been threaded along its entire axial length, but with one end region thereof formed with a circumferentially spaced array of either radially and longitudinally extending slits, or radially and spirally extending slits, so as to produce a plurality of internally threaded resilient fingers (or segments). One or more so-called garter springs have normally been coaxially mounted on such fingers so as to augment the inherent spring-biased compressive forces of the resilient fingers which maintain the latter in continuous contact with the threads of the lead screw.
In still another prior alternative design, an elongated drive nut has been formed with an intermediate section having a thin wall, with a circumferential array of longitudinally disposed slits formed therein, as well as in a front end section that is slightly tapered. This allows a variable degree of expansion of the drive nut body over an appreciable portion of the axial length thereof.
In all of such prior drive nut designs, the central bore, as previously mentioned, has been threaded along its entire axial length. As such, while prior drive nut versions may have a resilient section to minimize backlash by presenting a continuous "load" on the lead screw, the end-to-end internally threaded bore prevents the drive nut from being slightly tilted or skewed relative to the axis of the lead screw. Such movement is often desired in order to compensate for any bow in the lead screw, as well as for any misalignment thereof relative to the carriage guide rods.
Another approach to the problem of minimizing excessive or detrimental frictional forces between a drive nut and lead screw has been to purposely build-in a predetermined degree of backlash therebetween. It is appreciated from the foregoing, of course, that in such a case the drive nut would normally not be constructed with a segmented resilient section, as such a section is intended and employed to minimize backlash. Accordingly, prior backlash producing drive nuts have each typically taken the form of a conventional elongated, solid wall, tubular member with a threaded bore extending along the entire axial length thereof. Such a construction, however, even with loose tolerances, prevents any appreciable tilting or skewing of the drive nut relative to the axis of the lead screw.
An equally important problem that arises when a built-in degree of backlash is employed in a lead screw-drive nut assembly is the fact that a substantial degree of kinetic energy is necessarily established by the mass of the coupled carriage, together with any associated apparatus carried thereby, such as a print head. Such kinetic energy can establish substantially large, initial impact forces, as well as transient forces, between the lead screw and drive nut threads if not compensated for or absorbed in some way. These detrimental forces, of course, lead to a "bouncing" condition of the carriage (and print head) which has proven to be particularly troublesome in lead screw driven printers where the carriage is stepped from one character print column position to the next across the width of the platen.
The potential severity of force-induced bouncing of a stepped carriage resides in the fact that if all of the kinetic energy imparted by the drive nut-carriage assembly to the lead screw is not absorbed completely as it is established, there will be increased wear of the mating lead screw-drive nut threads, and the desired speed of travel of the carriage may be adversely affected.
Considered another way, the kinetic energy induced forces ideally should be absorbed at a rate which is equatable to the change in velocity of the drive nut-carriage assembly. Unfortunately, prior drive nuts have not been able to inherently, or as mounted on or coupled to the carriage, compensate for kinetic energy induced bounce forces, particularly in a stepped carriage mode of printer operation.